Remove unused code

Closes gh-47235

Author: snicoll

core/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ErrorProperties.java

@@ -114,28 +114,6 @@ public Whitelabel getWhitelabel() {
 		return this.whitelabel;
 	}
 
-	/**
-	 * Include Stacktrace attribute options.
-	 */
-	public enum IncludeStacktrace {
-
-		/**
-		 * Never add stacktrace information.
-		 */
-		NEVER,
-
-		/**
-		 * Always add stacktrace information.
-		 */
-		ALWAYS,
-
-		/**
-		 * Add stacktrace attribute when the appropriate request parameter is not "false".
-		 */
-		ON_PARAM
-
-	}
-
 	/**
 	 * Include error attributes options.
 	 */